Expression of the transferrin receptor gene during the process of mononuclear phagocyte maturation

The expression of transferrin receptors by blood monocytes, human alveolar macrophages, and in vitro matured macrophages was evaluated by immunofluorescence, radioligand binding, and Northern analysis, using the monoclonal anti-human transferrin receptor antibody OKT9, [¹²⁵I]-labeled human transferrin and a [³²P]-labeled human transferrin receptor cDNA probe, respectively. By immunofluorescence, the majority of alveolar macrophages expressed transferrin receptors (86 ± 3%). The radioligand binding assay demonstrated the affinity constant (K(a)) of the alveolar macrophage transferrin receptor was 4.4 ± 0.7 x 10⁸ M^-1, and the number of receptors per cell was 4.4 ± 1.2 x 10⁴,. In marked contrast, transferrin receptors were not present on the surface or in the cytoplasm of blood monocytes, the precursors of the alveolar macrophages. However, when monocytes were cultured in vitro and allowed to mature, >80% expressed transferrin receptors by day 6, and the receptors could be detected by day 3. Consistent with these observations, a transferrin receptor mRNA with a molecular size of 4.9 kb was demonstrated in alveolar macrophages and in vitro matured macrophages but not in blood monocytes. Thus, although blood monocytes do not express the transferrin receptor gene, it is expressed by mature macrophages, an event that probably occurs relatively early in the process of monocyte differentiation to macrophages.